why can't i bind ipv6 socket to a linklocal address

Question

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>

void error(char *msg)
{
  perror(msg);
  exit(0);
}
int main(int argc, char *argv[])
{
   int sock, length, fromlen, n;
   struct sockaddr_in6 server;
   struct sockaddr_in6  from;

   int portNr = 5555;
   char buf[1024];

   length = sizeof (struct sockaddr_in6);

   sock=socket(AF_INET6, SOCK_DGRAM, 0);
   if (sock < 0) error("Opening socket");

   bzero((char *)&server, length);
   server.sin6_family=AF_INET6;
   server.sin6_addr=in6addr_any;
   server.sin6_port=htons(portNr);

   inet_pton( AF_INET6, "fe80::21f:29ff:feed:2f7e", (void *)&server.sin6_addr.s6_addr);
   //inet_pton( AF_INET6, "::1", (void *)&server.sin6_addr.s6_addr);

   if (bind(sock,(struct sockaddr *)&server,length)<0)
       error("binding");
   fromlen = sizeof(struct sockaddr_in6);
   while (1) {
       n = recvfrom(sock,buf,1024,0,(struct sockaddr *)&from,&fromlen);
       if (n < 0) error("recvfrom");
       write(1,"Received a datagram: ",21);
       write(1,buf,n);
       n = sendto(sock,"Got your message\n",17,
                  0,(struct sockaddr *)&from,fromlen);
       if (n  < 0) error("sendto");
   }
}

when I compile and run the above code I got :

binding: Invalid argument

and if change to bind the ::1 and leave other thing unchanged in the source code, the code works! so could you tell me what's wrong with my code ? thanks in advance.

Answer 1

For link-local addresses, you also need to specify the scope ID of the network interface that is associated with the address... something like this:

server.sin6_scope_id = 5;   /* or whatever the scope ID is for the network interface you want to communicate over */

You can use getifaddrs() to find the various scope IDs available on your systems, and the network interfaces they correspond to.

(Yes, it's a pain... alternatively you might be able to append something like "%en0" to the end of the string you pass to inet_pton(), and inet_pton() might do the work for you... I'm not sure if inet_pton() handles that syntax or not)